The present invention relates to a developing apparatus applicable to an image forming apparatus which copies an image, based on image data obtained by optically reading an image of an object as brightness data.
For example, in a copying apparatus which uses electrostatic copying process, an image is copied by developing an electrostatic latent image corresponding to image data of a object to be copied, transmitted as brightness of light to a photosensitive material.
The photosensitive material is, for example, a drum-like photosensitive member in which a thin layer made of an inorganic or organic photosensitive material is formed on an outer circumference formed in a cylindrical shape.
A method used when developing an electrostatic latent image is such a method in which a toner is selectively applied as a visualizing agent to an electrostatic latent image on a photo-sensitive member by an electrostatic force between the toner and the electrostatic latent image.
A widely used method of supplying toner onto an electrostatic latent image on a photosensitive member is by two-component magnetic brush development in which a developing agent obtained by mixing a carrier for carrying the toner with the toner at a predetermined ratio being carried to the photosensitive material with use of a developing roller internally including a fixed magnet and having an outer circumferential surface arranged to be rotatable at a predetermined speed, thereby to apply only the toner onto the electrostatic latent image on the photosensitive member.
In the two-component magnetic brush development, at present, a carrier consisting of grains each having a small diameter, i.e., a small diameter carrier is used to improve the image quality.
However, the method of using a small diameter carrier has a problem that the carrier is applied together with the toner to the photosensitive material.
The carrier thus adhered to the photosensitive material may damage the surface of the photosensitive member and blades of any cleaning device when a toner image obtained by developing an electrostatic latent image is transferred to a recording paper sheet as a transfer medium.
Hence, a proposal has been made to an image forming apparatus in which a carrier collection device for collecting the carrier is provided in the vicinity of the photosensitive member, e.g., between the developing roller and the transfer device.
However, in the carrier collection device, a magnetic member capable of generating a particular magnetic field different from that generated by the developing roller, and the carrier is collected by the magnetic field generated by the magnetic member. Therefore, there is a problem that both magnetic fields generated by the developing device and by the carrier collection device complicatedly influence each other and the carrier is directly attracted to the collection device from the developing roller, depending on some conditions.
This phenomenon is caused when the magnetic force of the carrier collection device is greater than the magnetic force of the main pole of the developing roller and the carrier pole. Once this phenomenon has occurred, a problem appears in that the developing agent stays in the collection device and the efficiency of collecting the developing agent is greatly lowered.
Since the developing agent thus stays in the collection device, there occurs a problem that the developing agent is accumulated between the developing roller and the collection device, and the developing roller is locked so that the electrostatic latent image cannot be developed.